1. Field of the Invention
The present invention relates to a surface acoustic wave device having several types of IDTs on a piezoelectric substrate, and a method of producing such a surface acoustic wave device.
2. Description of the Prior Art
In recent years, to increase the utility of mobile communication devices, research has been done on a multi-band corresponding portable telephone having at least two communication systems. In addition, for portable telephones, a higher transmission frequency has been increasingly used.
Accordingly, with respect to a terminal which can use both of the communication systems for a 800 MHz band cellular phone and an at least 1.5 GHz band cellular phone, it is necessary to provide an RF band-pass filter for each of the two different frequencies.
The reduction of the number of components required for such a terminal, to reduce size and weight, has a limitation. Accordingly, one component having two filter functions is desirable.
For this reason, it has been proposed that several filter functions be provided on one piezoelectric substrate. A surface acoustic wave device has been proposed which has two types of electrodes such as IDTs or other suitable electrodes, having different film-thicknesses formed on one piezoelectric substrate so that the device can operate in two different frequencies.
A method of producing such a surface acoustic wave device has been proposed by Japanese Unexamined Patent Application Publication No. 10-190390, for example. Hereinafter, the method of producing a surface acoustic wave device will be described with reference to FIGS. 6A to 6E.
First, a conductive film 104a having a predetermined film thickness is formed on a piezoelectric substrate 101. On the piezoelectric substrate 101 having the conductive film 104a, a resist is provided. Next, the resist is exposed using a mask having a shielding portion corresponding to the pattern of the IDT of a first surface acoustic wave element, and then the resist is developed. Accordingly, the exposed portion of the resist is removed. As a result, the unexposed portion 102a of the resist corresponding to the IDT pattern of the first surface acoustic wave element remains.
The conductive film 104a is removed by etching, except for the portion of the conductive film corresponding to the resist 102a. Accordingly, the IDT 111 of the first surface acoustic wave element 110 is produced as shown in FIG. 6B.
Next, a resist is provided on the piezoelectric substrate 101 having the IDT 111 of the first surface acoustic wave element, and exposed using a mask which includes an opening portion corresponding to the IDT of a second surface acoustic wave element, and then the resist is developed whereby the exposed portion of the resist is removed. As a result, the resist 102b having an opening portion corresponding to the IDT of the second surface acoustic wave element is produced, as shown in FIG. 6C.
Next, as shown in FIG. 6D, a conductive film 104b having a smaller film-thickness than that of the IDT 111 of the first surface acoustic wave element 110 is disposed on the piezoelectric substrate 101 having the resist 104b. 
Finally, the resist 102b and the conductive film 104b disposed on the resist 102b are removed simultaneously. As shown in FIG. 6E, the IDT 121 of the second surface acoustic wave element 120, having a smaller thickness than the IDT 111 of the first surface acoustic wave element 110, is produced.
According to the above-described production method, the surface acoustic wave elements having different frequency characteristics are produced on the same piezoelectric substrate.
The surface acoustic wave device described above has a problem associated with the manufacturing method thereof. Specifically, when the film-thickness and/or the electrode finger width of the IDT deviate from their predetermined ranges so that desired frequency characteristics are not obtained after the formation of the IDT of the second surface acoustic wave element, the surface acoustic device is defective even though the previously produced frequency characteristic of the first surface acoustic wave element have been obtained.
That is, the first and second surface acoustic wave elements are formed on the same piezoelectric substrate to define a composite device. Accordingly, if only one of the surface acoustic wave elements has a frequency characteristic that deviates from a desired value, the device is rendered defective. Thus, the defective ratio is considerably higher than the defective ratios arising from surface acoustic wave devices wherein the first and second surface acoustic wave elements are formed on separate piezoelectric substrates.
Further, when a resist pattern is used to produce an IDT, the resist pattern is generally heated to enhance the adhesion thereof to a piezoelectric substrate and plasma-resistance. When this process is applied to the method of producing a surface acoustic wave device in which a plurality of surface acoustic wave elements having different frequency characteristics are included, a problem arises.
More specifically, when the resist pattern used to produce an IDT of a second surface acoustic wave element is heated after formation of a first surface acoustic wave element, a potential difference between the electrode fingers of the IDT in the first surface acoustic wave element is generated due to the pyroelectric properties of the piezoelectric substrate. The potential difference is discharged and pyroelectric breakdown is possible. Even if the discharge is not large enough to cause the pyroelectric breakdown, the discharge could destroy or distort the resist pattern leading to a short-circuit of the IDT of the first surface acoustic wave element during the formation of the IDT of the second surface acoustic wave element.
To overcome the problems discussed above, the preferred embodiments of the present invention provide a surface acoustic wave device in which surface acoustic wave elements with different frequency characteristics are provided on a piezoelectric substrate which has a greatly reduced rejection ratio and a greatly increased reliability, and a method of producing such a surface acoustic wave device.
A preferred embodiment of the surface acoustic wave device includes a piezoelectric substrate, a first surface acoustic wave element including at least one interdigital transducer on the piezoelectric substrate, and a second surface acoustic wave element including at least one interdigital transducer which is provided on the piezoelectric substrate and has a thickness that is different from that of the interdigital transducer of the first surface acoustic wave element. The second surface acoustic wave element has a frequency characteristic that is different from that of the first surface acoustic wave element. An insulating film is disposed on the first and second surface acoustic wave elements, and the thickness of the insulating film in the region of the first surface acoustic wave element is different from the thickness of the insulating film in the region of the second surface acoustic wave element.
With this unique structure and arrangement, the frequency characteristics of the two different surface acoustic wave elements are effectively adjusted.
Another preferred embodiment of the present invention provides a method of producing a surface acoustic wave device in which first and second surface acoustic wave elements are provided on a piezoelectric substrate, the method including the steps of forming an interdigital transducer of the first surface acoustic wave element on the piezoelectric substrate, the interdigital transducer having input-output terminals which are electrically connected by a short-circuiting electrode, providing a resist on the whole surface of the substrate where the interdigital transducer of the first surface acoustic wave element and the short-circuiting electrode are located, and heating the resist, removing the resist only on the area where the second surface acoustic wave element is to be located, forming a conductive film on the piezoelectric substrate, the conductive film having a thickness that is different from a thickness of the interdigital transducer of the first surface acoustic wave element, patterning the conductive film to produce an interdigital transducer of the second surface acoustic wave element by a lift-off method, cutting the short-circuiting electrode to electrically disconnect the input-output terminals of the interdigital transducer of the first surface acoustic wave element from the second surface acoustic wave element, forming an insulating film on the interdigital transducers of the first and second surface acoustic wave elements, and decreasing the thickness of the insulating film to adjust frequency characteristics of the first and second surface acoustic wave elements.
Accordingly, the IDT electrode of the first surface acoustic wave element is prevented from being short-circuited after the second surface acoustic wave element is lifted-off. In addition, the defective ratio of the composite element can be reduced by forming an SiO2 film on the first and second surface acoustic wave elements.
The method of another preferred embodiment of the present invention may further include the steps of measuring the frequency characteristics of the first and second surface acoustic wave elements by wafer probing prior to the step of adjusting the frequency, wherein the step of decreasing the thickness of the insulating film is performed so that the thickness of the insulating film in a region of the interdigital transducer of the first surface acoustic wave element is different from the thickness at the region on the interdigital transducer of the second surface acoustic wave element.
Further, the insulating film produced by the insulating film forming step may have a predetermined thickness such that one of the first and second surface acoustic wave elements has desired frequency characteristics, and the step of decreasing the thickness of the insulating film is performed by etching only in the region of the other of the first and second surface acoustic wave elements.
Alternatively, the step of decreasing the thickness of the insulating film may include the steps of decreasing the thickness of the entire insulating film such that one of the first and second surface acoustic wave elements has desired frequency characteristics, measuring frequency characteristics of the other of the first and second surface acoustic wave elements to determine the desired thickness of the insulating film for the other of the first and second surface acoustic wave elements to produce the desired frequency characteristics, and decreasing the thickness of the insulating film only in the region of the other of the first and second surface acoustic wave elements to produce the desired frequency characteristics based on the desired thickness determined by the measuring step.
Accordingly, the frequency characteristics of the first and second surface acoustic wave elements can be effectively adjusted.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments of the present invention below with reference to the attached drawings.